[1641] Add missing #include of missing header file

"string.h" is requested to define memset().

src/lib/dns/rdata/generic/detail/nsec_bitmap.cc

@@ -22,6 +22,7 @@
 #include <sstream>
 #include <vector>
 #include <stdint.h>
+#include <string.h>
 
 using namespace std;